Ion source of a duo-plasmatron



Jan. 5, 1965 H. WERNER 10N SOURCE oF A Duo-PLASMATRON 3 Sheets-Sheet 1 Filed July 20, 1960 Lind Jan. 5, 1965 H. WERNER 10N SOURCE oF A Duo -PLASMATRON 5 Sheets-Sheet 2 Filed July 20, 1960 lNvENToR #5f/vz Wam/5P Jan. 5, 1965 H. WERNER 10N SOURCE oF A DUo-PLAsMATRoN 5 Sheets-Sheet 5 Filed July 20, 1960 ATTO RN EY:

United States Patent O 3,164,739 ION SOURCE F A DUO-PLASMATRON Heinz Werner, ibi-erden, Germany, assigner to VEB Vakutronilr, Dresden, Germany Filed July 2l), 196%, Ser. No. 44,654 9 Ciaims. (Ci. 313-161) This invention relates generally to ion sources and has particular reference to improvements in the ion source of the type known as a Duo-Plasmatron.

In the conventional embodiment of a Duo-Plasmatron ion source the plasma density of an electrical discharge is doubled by means of an unhornogeneous magnetic iield produced by a magnetic pole piece lens. A magnetic winding conventionally produces the required magnetic flux. A filament in the magnetic pole piece lens is adjusted to a length such that the discharge mechanism in the vicinity of the double layer of the plasma jet is left unaffected by the associated magnetic field of the lens. In practice, for any particular predetermined length of the filament, the current in the magnetic winding is experimentally regulated so that optimum density is obtained by means of the unhomogeneous magnetic fleld of the lens Without varying the double boundary jet.

A known development of the Duo-Plasmatron utilizes a permanent magnet instead of the magnetic winding, so that the electrical members required for energizing the winding and for maintaining it constant may be avoided. This, however, does away with one desirable possibility of controlling the device. The practical operation of such ion sources is made considerably more diicult thereby and it may even be made impossible. This is of particular concern to the optical focusing and other features of adjustment and adaptation.

In addition to these important disadvantages of a permanent magnet ion source, conventional embodiments using permanent magnets are liable to still further deficiencies. Thus, in View of the high thermal load, the central water cooling of the magnetic pole faces is accommodated within ferromagnetic material, which results in a significant magnetic diminution for the axially magnetized annular magnets which are usually used. The latter are made of Maniperm, a ceramic material. Only a small fraction of the entire magnetic flux, which is actually produced, is thus available for producing of the unhomogeneous field between the operating pole faces.

Overcoming the decrease in magnetic force, by the use of non-ferromagnetic material, such as steel (eg. grade V2A), copper or other non-ferrous materials, is difficult because of the adaptation and the above-mentioned thermal load, and because of the need for simple manufacturing procedures. The use of ferromagnetic steel, however, acts as a magnetic shunt. Avoiding the magnetic shunt by the use of the cited ceramic material Maniperm is very difficult and does not cool the pole shoe in such a manner as is necessary because of the decrease of heat conductivity. However, by applying the ferromagnetic material for the wall of the cooling system, a considerable magnetic diminution results.

In addition, the following diiiiculty exists. In order to maintain the resulting electrical potential difference of the magnitude of 100 volts between the magnetic faces or shoes during the operation of the ion source, it is necessary to provide an air gap serving as an insulator. The air gap further reduces the field strength in the vicinity of the pole pieces, which strength has already been materially reduced by the above-mentioned magnetic diminution.

Finally, in order to maintain within reasonable bounds the unusually intense heat generated in one part of the axially magnetized annular magnet during operation and alegria uatented Jan. 5, 1&65

'ice

to diminish the possibility of exceeding the Curie point of 450 C. for permanent magnetaanother air gap is provided between the pole face and the ring magnet. This latter air gap again, undesirably, reduces the possible field strength of the pole face magnet in the vicinity of the pole face lens.

As a result of these numerous, collectively weakening, air gaps, which already exist in the overall arrangement of the operating air gap of the pole face lens when using a permanent magnet, the ability to provide the optimum conditions for direction and strength of the unhomogeneous magnetic field of the pole face lens is generally diminished.

The disadvantages of the known arrangements may be summarized as follows: The requisite and also attainable magnetic field strengths in the vicinity of the pole shoe lens do not become eective; the desired doubling of the density is not obtained by the arrangements, and the desiredradiation currents and emission current densities are missed by a wide margin. To again replace the permanent magnets by means of magnetic windings would, on the other hand, be a backward step.

It is an object of this invention to provide an improved Duo-Plasmatron.

It is a further object of the invention to provide a Duo- Plasmatron ion source using permanent magnets wherein the plasma density is adjustable.

It is a still further object of the invention to provide a Duo-Plasmatron ion source using permanent magnets which is easily adjustable from outside the apparatus by means of adjustment of air gaps within the apparatus.

Other objects and advantages of the invention will become apparent from the following description.

In accordance with the invention, magnetic shunts are arranged next to the operating magnetic ilux or magnetic reluctances are arranged in the path of the magnetic ux. The shunts are preferably accessible from the outside and adjustable in operation. By means of the shunts the unhomogeneous magnetic field of the pole face lens may be regulated to any desired value, and particularly to the optimum value.

The invention permits elimination of undesirable supplementary air gaps, thereby making possible the use of the desirable, radially magnetized permanent magnets whose production, preferably from ceramic and from ferromagnetic materials, and permits utilization of the insulating properties of those materials. The dissipation of heat can be made more effective. The invention minimizes the disadvantages of a conventional Duo-Plasmatron, by elimination of the electromagnet which was previously utilized for producing the necessary magnetic flux, and replacement of the electromagnet by a permanent magnet. The invention further allows adjustment of the magnetic flux produced by the permanent magnet to optimum conditions by provision of variable series and/or shunt members in the magnetic circuit. The adjustment of the magnetic flux was previously accomplished by controlling the current in the now discarded electromagnet.

Other objects and many of the attendant advantages of the invention will be readily appreciated as the same become better understood by reference to the following detailed description, when considered with the accompanying drawings, wherein:

FIG. 1 is a sectional view exemplifying the structure of former Duo-Plasmatron devices;

FIG. 2 is a sectional view similar to FIG. l of a Duo- Plasmatron which embodies the principles of the invention;

FIG. 2a is a partial showing of the apparatus in FIG. 2, illustrating the use of an electromagnet;

Vwinding of an associated electromagnetic core.

h shoelens 1, 4, while the double-arrow line alegres Y air-'gap adjustment is shown in a diferent position;

FIG. is a generalized schematic representation of an electrical and magnetic circuit illustrating the connections and magnetic circuit of the invention; and

FIG. 6 shows, in a partial View of FIG. l, the path `of the desired magnetic ilux shown by a double arrow.

-The magnetic adverse' shunt diminishing the inhomogeneous held of the pole shoe lens is marked by va singlev Yarrow line.V

h Referring now to FIGS. 1-5, wherein like numerals vdesignate like parts throughout, a hot cathode 16 is installed in the usual discharge chamber 12, wherein the pressureis reduced. Numeral 9 denotes an outer hous= ing of thel Duo-Plasmatron. An energizing permanent magnet 3 surrounds the discharge chamber 12 and terinitiates irl an unhomogeneus pole piece lrcooled by a waterA jacket 2 (shown schematically). The discharge chamber' 12 is provided with a vacuum Apump (not.

shown). The auxiliary 'gas required to. produce 'the plasma is fed into the discharge chamber 12 through a line (not shown), or by means of the aforementioned i Vr'vacuum pump. Another pole piece 4, positioned adjacent the pole piece 1 to form a pole piece lens,'acts as 'anemission anode; andan accelerating electrode 14, ywhich is suitably connected to a liv. voltage sourcey as shown, forms an additional chamber I6. A third chamber 1S is formed within the accelerationV electrode. A A hot cathode low-voltagecdischarge is used to produce Va plasma inthe evacuated chamber. The plasma is very'highly compressed by the pole piece lens 1, 4 and 'particles escape through ian extractor hole 20 drilled in the pole piece or anode 4. The particlesv are drawn olf by the accelerating electrode 14. The degree ofcornpresfsion of the plasma and the density of particlesV drawn ot depend on the value of the iiu'x of the magnetic lens. :'Normally the pole piece lens 1, 4 may be controlled to provide an' optimum held by regulation of current in a The permanent magnet 3 as shown in FIG. l normally cannot concentrate the plasma because the magnetic lield of the pole face lens cannot be controlled thereby to its optimum'condition.

i In the conventional embodiments of the Duo-Plasmatron a voltage difference of approximately 1GO volts must 4.exist between the Afaces 1 and. 4. Air gaps such as 5 and `6 in FIG. 1 are kconventionally used to provide electrical insulation betweenv parts. lIn FIG. 6, corresponding to a portion of FIG. l, the magnetic ux path is illustrated. The single-arrow line denotes the magnetic adverse shunt `which diminishes the inhomogeneous iield ofthe pole indicates the desired path of the magnetic ux. Y

In accordance with the invention, instead of the ring lmagnet 3 (FIG. 1'), which is made of axially or longitudinally magnetized Maniperm(a ceramic), the magnet 3a (FIGS. 2 and 4) is radially magnetized. The water cooling 2, accommodated within the iron body in the old construction (FIG. 1) which formsa magnetic shunt and impairs the function of the original magnet, is now l formed as an inner ring for themagnetic pole piece 1 (FIGS. 1, 2 and 4). Y

Further, in accordance with the invention, the second pole piece 4 (FIGS. 2 and 4) which, together with the pole piece 1, constitutes the pole piece lens proper, is extended to the pole of the permanent magnet in ferromagnetic material, .the pole being situated outside in radial direction of the permanent magnet (FIGS. 2 and 4), without the necessity of providing air gaps as, for example, those at 5 and 6 (FIG. l) which had been absolutely necessary until now. The pole shoe partAS (FIG. l), hitherto necessary, can thus be dispensed with. The magnetic paithconsequently connects the magnet 3a through the housing Wall 9 with the pole piece 4. Also, the second pole shoe par-.t 28 (FIG. l) may prefenably bev omitted in the inventive embodiments. Y rl`he required electric insulation between the pole faces 1 and 4 (FIGS. 2, 4), having dilerent potentials, is accomplished by the permanent magnet 3a pro-per with its specic resistance of 108 ohms per centimeter. This electrical bypass to the working resistance situated between the pole faces and having several hundred ohms, which bypass is accomplishedby the magnet, has no importance from the electrical point of view.V

As a furtherfeaiture ofthe invention, the tempera- 'ture of the magneticrmaterial is maintained below the maximum permissible temperature, which is slightly below the Curie point ofthe magnetic material, by contacting the magnetic material only by Water cooled or air cooled surfaces (such as the jacket 2) or by poorly conductive materials, such as Ceramics, as shown at 11 in FIGS. 2 and 4.

Finally, outside the vacuum portion of the ion source, there are arranged one or more magnetic shunt connections 7 (FIGS. 2 and 3) or( alternately variable magnetic series resistances (FIG. 4) which are accessible from the outside and aligned with the direction of the main magnetic iluxwithin the magnet, The shunts are controlled by threaded magnetic bolts 22 which may be axially adjusted to vary the magnetic shunt connection. In contrast to these hitherto known embodiments which could not be controlled, the connections according to the invention are adjustable so that the asymmetrical field produced'by the pole face lens displays the optimum value and does not adversely Yeffect the discharge mechanism or the plasma ow.

For the purpose of maintaining existing potential differences between the pole faces 1 and 4 (FIGS. 2, 4)

wherein the material of the magnet is not sufficiently insulated, an air gap can, however, be provided for eit- 'ample, on the magnetic shunt '7' (FIGS. 2 `and 3), said gap being merely a fraction of those hitherto applied.

By the above-mentioned means it is possible to make the ring magnets 3a, which are made of Maniperm, even tiatter. In this manner, while using a permanent magnet, the same emission density may be obtained which was hitherto only available with electromagnets.

The permanent magnet mayV alsobe replaced completely or partly by an electromagnet, as shown at 3b in FIG. 2a. An advantageous embodiment therefore is a similar arrangement asdescribed for the permanent magnet. This provides for improved control and adjustability of the desired magnetic field. f For advantage-ous further increase in density of the effective' field ',strengths, several magnetic orelectrical fields may be utilized (not shown).

As shown in FIG. 3, it is also possible to insert an adjustable variable magnetic series resistance arranged in the direction of the main magnetic flux, inY place of the magnetic shunt.

FIG. 4 shows such a variable series resistance. An outside ring 24 formingpart of (the housing, inside of which the radially magnetized ring magnet-3a is placed, consists of ferromagnetic material. The magnetic ux to the pole shoe 4 is provided by the metal surrounding the water cooling system 2 and the ring 24. This ring has two parts which are separated by a ferromagnenc ring 23 acting as an air gap for the magnetic flux. This air gap maybe varied (diminished) by displacing a ring 25 which is made of ferromagnetic material. The mechanism for displacing the ring y25 in several steps or without steps is not shown. Asa-matter of example, it may consist of 4a pegV 26 traversing the ring V25 and riding in a helical groove 27 yprovided in the housing ring 24. Thus, manual rotation tof ring 25 will'shift the latter bee' 5.3 tween the position shown in FlG. 4 (air gap 23 uncovered) and the position of FG. 4d wherein the ferromagnetic ring 25 overlaps the non-ferromagnetic ring 23. This arrangement provides iine adjustment of the air gap in the llux path between the ring magnet 3a and the pole piece 4. By means of the magnetic series resistance tbe desired and optimum value of inhomogeneous magnetic field of the pole Jface lens may be regulated. lt should be understood, of course, tbat tbe foregoing disclosure relates only to preferred embodiments of the invention, and that it is intended to cover all changes and modifications ot' tbe examples described, which do not constitute departures from tbe spirit and scope of the invention as set forth in the appended claims.

ln respect to FlG. 4 it should be mentioned that instead of variable magnetic sbunts, as mentioned above, also one or more variable magnetic series resist-ances are applicable without shunts, or in connection with magnetic shunts,

What is claimed is:

l. An ion source wherein an electrical discharge between two magnetic members having a potential difference is spatially limited by means of a magnetic pole piece lens formed by said members, one of said members being inwardly of the other member, and wherein a permanent magnet is included in the magnetic circuit to provide the magnetic ux for said lens, the improvement comprising a movable magnetic member in said magnetic circuit, means for progressively moving said movable magnetic member from a position wherein the flux of said circuit is maximum to a postion of minimum magnetic flux.

2. An ion source comprising an annular radially magnetized ceramic permanent magnet, a first magnetic member extending from the inner pole of said permanent magnet for forming a rst piece of a magnetic pole piece lens, a second magnetic member extending toward said irst member for forming a second piece of said pole piece lens, chamber means for enclosing the volume between said members, means for producing a potential dierence between said members, means for causing an electrical discharge in the vicinity of said members, whereby said members may control said discharge magnetically, a movable magnetic member positioned adjacent said permanent magnet, means outside said chamber for adjusting the position of said movable magnetic member so as to adjust the flux density of said pole piece lens.

3. An ion source as in claim 2, wherein said magnetic members are annular and coaxial.

4. An ion source comprising an annular radially magnetized magnet, a first magnetic member extending from the inner pole or" said magnet for forming a first piece of a magnetic pole piece lens, a second magnetic member extending toward said irst member for forming a second piece or" said pole piece lens, chamber means for enclosing the volume between said members, means for producing a potential difference between said members, means for causing an electrical discharge in the vicinity of said members, whereby said members may control said discharge magnetically, a movable magnetic member positioned adjacent said permanent magnet, means outside said chamber for adjusting the position of said movable magnetic member so as to adjust the ilux density of said pole piece lens.

5. An ion source as in claim 4, wherein said magnet is an electromagnet.

6. An ion source as in claim 4, wherein the magnetic llux between said li'st and second pieces is separated as to its potential by said annular magnet` 7.. An ion source as in claim 4, further comprising a magnetic shunt member extending from one pole of said annular magnet to the other pole, means for progressively removing said shunt and leaving an air gap between said poles, for varying the magnetic flux at said pole piece lens to an optimum value.

8. An ion source as in claim 4, wherein said lluX density is adjusted by a variable magnetic series resistance accessible from the outside of said chamber means.

9. An ion source as in claim 4,. wherein said annular magnet is positioned coaxially to said electrical discharge.

References Cited in the tile ofthis patent UNlTED STATES PATENTS 2,826,709 Von Ardenne Mar` 1l, 1958 

1. AN ION SOURCE WHEREIN AN ELECTRICAL DISCHARGE BETWEEN TWO MAGNETIC MEMBRS HAVING APOTENTIAL DIFFERENCE IS SPATIALLY LIMITED BY MEANS OF A MAGNETIC OLE PIECE LENS FORMED BY SAID MEMBERS, ONE OF SAID MEMBERS BEING INWARDLY OF THE OTHER MEMBER, AND WHEREIN A PERMANENT MAGNET IS INCLUDED IN THE MAGNETIC CIRCUIT TO PROVIDE THE MAGNETIC FLUX FOR SAID LENS, THE IMPROVEMENT COMPRISING A MOVABLE MAGNETIC MEMBER IN SAID MAGNETIC CIRCUIT, MEANS FOR PROGRESSIVELY MOVING SID MOVABLE MAGNETIC MEMBER FROM A POSITION WHEREIN THE FLUX OF SAID CIRCUIT IS MAXIMUM TO A POSTION OF MINIMUM MAGNETIC FLUX. 